Balloon catheters are used to perform various medical procedures within the body. One example is a percutaneous transluminal coronary angioplasty (PTCA). To perform a PTCA, a physician guides a balloon catheter through the vascular system over a guide wire until the catheter's balloon is positioned across a thrombus, commonly referred to as a lesion, that has reduced or completely blocked blood flow through a vessel. Once placed, the balloon is inflated radially, or that is, outwardly, to a pre-determined diameter, for example 9 millimeters, to reestablish blood flow through the vessel.
To maneuver a distal end of the balloon catheter through the vascular system to treat a lesion that has a very small opening requires the balloon catheter to have a small crossing profile, which is the largest cross-sectional diameter of the balloon portion of the balloon catheter. One portion of the crossing profile is the diameter of the catheter's shaft, which must be large enough to allow a guide wire to pass freely through a guide wire lumen in the shaft in a longitudinal direction. Catheters that use a guide wire with a larger diameter will have a larger catheter shaft and vice-versa. Existing catheters typically use guide wires that have cross-sectional diameters in the range of fourteen thousandths of an inch (0.014) up to 35 thousandths of an inch (0.035). Smaller guide wires make it easier to maneuver the balloon catheter through the body. Consequently, many doctors that perform cardiac procedures prefer using balloon catheters with a 0.014 inch guide wire.
Another portion of a balloon catheter's crossing profile is the thickness of the folded balloon, particularly at the balloon's folded cone and waist portions. Catheter balloons typically have a body portion near the center of the balloon, which is the portion of the balloon that expands. The balloon's waist portions are located at the balloon's proximal and distal ends, and the cone portions are located between the balloon's body portion and the waist portions. Typically, the wall thickness of the entire balloon is restricted to the dimensions of the original tubing material, or parison, from which the balloon is formed. Thus, an overall reduction in the wall thickness of the parison to reduce the thickness of the cone and waist portions will also result in a similar reduction of the wall thickness of the parison in the body portion. A reduced body portion wall thickness, however, limits the diameter to which the balloon may be inflated while maintaining a satisfactory hoop strength, which is a measure of a balloon's ability to withstand internal pressure. Consequently, there have been continuing efforts to form balloons made of materials suitable for cardiac procedures, such as Nylon 12, PEBAX, PET, and PEEK, that have large inflation diameters, for example 9 millimeters, and small crossing profiles.
One method of forming balloons with body portions that have a greater wall thickness and waist portions that have a reduced wall thickness is to physically remove, or grind away, the undesired thickness of the waist portions. This method may be performed, for example, by using a laser ablation process utilizing UV excimer lasers or ultrashort lasers. The physical removal of the wall, however, has not been able to produce a balloon that has an inflation diameter of greater than six millimeters for use on a catheter having a 0.014 inch guide wire.
The wall thickness of a polymer tube may also be changed by a stretching, or “drawing down,” process. Using this process, the wall thickness of the tubing may be locally reduced in the cone and waist portions of the balloon. Heating is advantageous during the stretching process because polymers have certain thermophysical characteristics that cause them to soften and flow upon heating. Thus, with the use of heat, it is possible to target a specific portion of a polymer tube for stretching. Various heating methods are used to heat polymer tubes for stretching. These methods include passing a polymer tube through a heated ring and putting the tube in a hot oil or hot water bath. These methods, however, result in a large heating area and do not provide the ability to target small areas of the polymer tube for stretching. For example, the heating area of a heated ring is typically greater than five millimeters along the axial direction. The heating area of a hot oil or hot water bath is even greater. Thus, these methods that essentially provide heat from the outside of the tube are not suitable to vary the amount of stretching of a coronary balloon, which is typically one to two centimeters in length (the cone and waist portions being only a few millimeters in length). Moreover, it takes several seconds to change the amount of heat applied to the polymer tube using these methods.